In which of the following reactions, chlorine acts as an oxidising agent?
$(i)$ $CH_3CH_2OH + Cl_2 \longrightarrow CH_3CHO + HCl$
(ii) $CH_3CHO + Cl_2 \longrightarrow CCl_3CHO + HCl$
(iii) $CH_4 + Cl_2 \stackrel{hv}{\longrightarrow} CH_3Cl + HCl$
The correct answer is

$100 \, mL$ of a sample containing calcium oxalate $[CaC_2O_4]$ is dissolved in an acidic solution. It required $20 \, mL$ of $1 \, M \, KMnO_4$ for complete oxidation. Find the mass of $Ca^{2+}$ ions (in grams) present in the sample. [Atomic mass of $Ca = 40$]

Justify that the following reactions are redox reactions:
$(a) \ CuO_{(s)} + H_{2(g)} \to Cu_{(s)} + H_2O_{(g)}$
$(b) \ Fe_2O_{3(s)} + 3CO_{(g)} \to 2Fe_{(s)} + 3CO_{2(g)}$
$(c) \ 4BCl_{3(g)} + 3LiAlH_{4(s)} \to 2B_2H_{6(g)} + 3LiCl_{(s)} + 3AlCl_{3(s)}$
$(d) \ 2K_{(s)} + F_{2(g)} \to 2K^+F^-_{(s)}$
$(e) \ 4NH_{3(g)} + 5O_{2(g)} \to 4NO_{(g)} + 6H_2O_{(g)}$

On treating $SO_2$ with an aqueous solution of $KMnO_4$,the manganese ion reduces to

Which of the following reactions represents a redox reaction involving water?

Which of the following can act as both a reducing and an oxidising agent?